What is "thecostraca"

Thecostraca is a subclass of marine invertebrates containing about 1,320 described species. Many species have planktonic larvae which become sessile or parasitic as adults.

The most important subgroup are the barnacles (infraclass Cirripedia), constituting about 1,220 known species.

The subgroup Facetotecta contains a single genus, Hansenocaris, known only from the tiny planktonic nauplii called "y-larvae". These larvae have no known adult form, though it is suspected that they are parasites, and their affinity is uncertain: some researchers believe that they may be larval tantulocaridans (no larval tantulocaridans are known, so this would solve two puzzles at once).

The group Ascothoracida contains about 100 species, all parasites of coelenterates and echinoderms.